Add trend plot axis labels when widget controls are missing

What you need before editing

Have the following available:

• The editable HMI screen and its runtime preview.
• The trend configuration, including the data source assigned to each trace.
• The quantity and engineering unit represented by each axis.
• The intended screen dimensions and any supported resizing behavior.
• A basic text object that can display a caption independently of the trend.

First inspect the widget's documented properties. Look for axis captions, axis titles, and chart headings. Tick labels are different: they show scale values, not necessarily the quantity or its unit.

If the required caption controls are absent, use adjacent text objects. Do not change working tag bindings just to make an axis name appear. Avoid undocumented modifications to the widget's internal rendering, because those changes can be lost when the widget redraws or the application is updated.

Decide what each caption must say

Write down the meaning of each axis before placing text. Do not infer it from the shape of a curve.

For the horizontal axis, establish whether the plot shows clock time, elapsed time, sample number, or another measured variable. A historical trend and an XY plot can look similar while giving the horizontal position different meanings.

For the vertical axis, confirm both the quantity and the engineering unit. Check any scaling between the source value and the displayed value. A raw sensor count is not automatically a temperature, pressure, or percentage.

Useful caption patterns include:

• Elapsed time (s) for a duration expressed in seconds.
• Temperature (°C) for temperature values displayed in degrees Celsius.
• Pressure (kPa) for pressure values displayed in kilopascals.

These are examples, not substitutes for checking the configuration. If a percentage is shown, establish what its reference represents. If clock time is shown, make the time basis clear where different time zones could cause confusion.

Keep the plot title separate from these definitions. The title identifies what the plot is about, such as a process area or measurement group. The axis captions explain how to read its coordinates.

Add the captions and title

1. Preserve the working screen. Save a recoverable project version using the editor's supported workflow. Note the existing plot position, size, and bindings so you can distinguish layout changes from data changes.

2. Reserve space around the plot. Leave room below the horizontal tick values, beside the vertical tick values, and above the chart. If space is limited, adjust the surrounding layout rather than placing text over the plotted data or clipping the scale.

3. Add the horizontal caption. Insert a text object and enter the confirmed quantity and unit. Position it beneath the horizontal axis, visually centered on the plotting area rather than on the entire widget if the widget includes a wide legend or side panel.

4. Add the vertical caption. Place another text object beside the intended vertical scale. Use rotated text only if the editor supports it and the result remains readable. Otherwise, put a short horizontal caption near that scale with enough separation to make the association clear.

5. Add the plot title. Place a separate text object above the chart. Give it a distinct position and text style so it cannot be mistaken for an axis caption or a trace name. Keep it short enough that it does not crowd the chart at the smallest supported layout.

6. Keep the objects together. Where the editor supports containers, grouping, or anchors, use those features to maintain the relationship between the plot and its captions. Confirm how those features behave during resizing. Grouping objects for editing does not necessarily make their runtime layout responsive.

7. Review the complete chart. Check the title, axis captions, tick values, and legend together. Remove duplicate wording that adds clutter, but retain the information needed to identify quantities, units, and individual traces.

Do not assume the text now follows changes to the trend configuration. Unless you explicitly configure a supported binding, a separate text object is simply a caption maintained independently of the plotted data.

Handle multiple scales and changing units

A single vertical caption is suitable only when it accurately describes the traces that use that scale. If traces represent different quantities, use the widget's supported separate scales or separate plots. Do not place unlike measurements under one caption merely because their numeric ranges happen to fit.

When a chart has a vertical scale on each side, put each caption beside its own scale. Use trace names or a legend to show which traces belong to which scale. Color can support that distinction, but should not be the only clue.

Changing units need extra care. If the application can switch the displayed values between unit systems, fixed text can become wrong. Either bind the caption to the same selected-unit state through documented features, or keep the chart in one clearly identified unit system. Updating only the caption without converting the values is equally misleading.

A change to the time basis also requires a caption review. Switching from elapsed time to timestamps changes the meaning of the horizontal axis, even if the chart occupies exactly the same space.

Check the result in runtime

Open the screen through its normal runtime path, not only in the layout editor. Confirm all of the following:

• Each caption sits beside the correct axis.
• The title is visibly separate from the axis captions and legend.
• Tick values remain visible, including minus signs and longer values.
• Captions do not overlap the trace area, controls, or neighboring charts.
• Text remains readable at the intended viewing position.
• Each displayed unit matches the configured scaling and data source.

Check every supported screen size and any permitted zoom or resize state. If the whole page is being scaled or clipped inside an embedded browser, use the checks for kiosk browser sizing in an HMI window before compensating with individual text positions.

Validate the data separately using an approved reference or a safe development environment. Do not manipulate a running process solely to make the curve move. Where the runtime supports history navigation or range changes, check that those actions do not cause tick values or controls to collide with the captions.

Keep captions correct after changes

Treat the text objects as part of the chart configuration during maintenance. Review them whenever you change a data source, engineering-unit conversion, axis assignment, time basis, or plot size.

If a caption stays behind after the chart moves, correct its layout relationship. If the caption looks right but the trace is missing or implausible, investigate the data path independently. Adding text does not repair acquisition, communication, or scaling faults.

For a fixed chart, separate text objects can be a straightforward solution. For a reusable chart with changing quantities, units, and scales, a widget with native caption support may be the better choice. The deciding question is whether the displayed words remain tied to the meaning of the plotted values throughout the screen's supported states.

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